6 Moose::Cookbook::Recipe2 - A simple B<BankAccount> example
15 has 'balance' => (isa => 'Int', is => 'rw', default => 0);
18 my ($self, $amount) = @_;
19 $self->balance($self->balance + $amount);
23 my ($self, $amount) = @_;
24 my $current_balance = $self->balance();
25 ($current_balance >= $amount)
26 || confess "Account overdrawn";
27 $self->balance($current_balance - $amount);
30 package CheckingAccount;
35 extends 'BankAccount';
37 has 'overdraft_account' => (isa => 'BankAccount', is => 'rw');
39 before 'withdraw' => sub {
40 my ($self, $amount) = @_;
41 my $overdraft_amount = $amount - $self->balance();
42 if ($self->overdraft_account && $overdraft_amount > 0) {
43 $self->overdraft_account->withdraw($overdraft_amount);
44 $self->deposit($overdraft_amount);
50 In the first recipe we showed how to build basic Moose classes
51 whose attributes had various accessor schemes and built in
52 type constraints. However our objects were very data-oriented,
53 and did not have many behavioral aspects to them (i.e. - methods).
54 In this recipe, we will expand upon the concepts from the first
55 recipe and give a more realistic scenario of more behavior
58 We are using an example of a bank account, which has a standard
59 account (you can deposit money, withdraw money and check your
60 current balance), and a checking account which has optional
61 overdraft protection. The overdraft protection will protect the
62 owner of the checking account by automatically withdrawing the
63 needed funds from the overdraft account to ensure that a check
66 Now, onto the code. The first class B<BankAccount> introduces a
67 new attribute feature, that of a default value.
69 has 'balance' => (isa => 'Int', is => 'rw', default => 0);
71 This tells is that a B<BankAccount> has a C<balance> attribute,
72 which is has the C<Int> type constraint, a read/write accessor,
73 and a default value of C<0>. This means that every instance of
74 B<BankAccount> that is created will have it's C<balance> slot
75 initialized to C<0>. Very simple really :)
77 Next come the methods. The C<deposit> and C<withdraw> methods
78 should be fairly self explanitory, they are nothing specific to
79 Moose, just your standard Perl 5 OO.
81 Now, onto the B<CheckingAccount> class. As you know from the
82 first recipe, the keyword C<extends> sets a class's superclass
83 relationship. Here we see that B<CheckingAccount> is a
84 B<BankAccount>. The next line introduces yet another new aspect
85 of Moose, that of class based type-constraints.
87 has 'overdraft_account' => (isa => 'BankAccount', is => 'rw');
89 Up until now, we have only had C<Int> type constraints, which
90 (as I said in the first recipe) is a built-in type constraint
91 that Moose provides for you. The C<BankAccount> type constraint
92 is new, and was actually defined at the moment we created the
93 B<BankAccount> class itself. In fact, for every Moose class that
94 you define, a corresponding type constraint will be created for
95 that class. This means that in the first recipe, a C<Point> and
96 C<Point3D> type constraint were created, and in this recipe, both
97 a C<BankAccount> and a C<CheckingAccount> type constraint were
98 created. Moose does this as a convience for you so that your
99 class model and the type constraint model can both be kept in
100 sync with one another. In short, Moose makes sure that it will
103 Next, we come to the behavioral part of B<CheckingAccount>, and
104 again we see a method modifier, but this time we have a C<before>
107 before 'withdraw' => sub {
108 my ($self, $amount) = @_;
109 my $overdraft_amount = $amount - $self->balance();
110 if ($self->overdraft_account && $overdraft_amount > 0) {
111 $self->overdraft_account->withdraw($overdraft_amount);
112 $self->deposit($overdraft_amount);
116 Just as with the C<after> modifier from the first recipe, Moose
117 will handle calling the superclass method (in this case the
118 C<BankAccount::withdraw> method). The C<before> modifier shown
119 above will run (obviously) I<before> the code from the superclass
120 with run. The C<before> modifier here implements the overdraft
121 protection by first checking if there are enough available
122 funds in the checking account and if not (and if there is an overdraft
123 account available), it transfers the appropriate funds into the
126 As with the method modifier in the first recipe, there is another
127 way to accomplish this same thing using the built in C<SUPER::>
128 pseudo-package. So the above method is equivalent to the one here.
131 my ($self, $amount) = @_;
132 my $overdraft_amount = $amount - $self->balance();
133 if ($self->overdraft_account && $overdraft_amount > 0) {
134 $self->overdraft_account->withdraw($overdraft_amount);
135 $self->deposit($overdraft_amount);
137 $self->SUPER::withdraw($amount);
140 The benefits of taking the method modifier approach is that the
141 author of the B<BankAccount> subclass does not need to remember
142 to call C<SUPER::withdraw> and to pass it the C<$amount> argument.
143 Instead the method modifier assures that all arguments make it
144 to the superclass method correctly. But this is actually more
145 than just a convience for forgetful programmers, it also helps
146 isolate subclasses from changes in the superclasses. For instance,
147 if B<BankAccount::withdraw> were to add an additional argument
148 of some kind, the version of B<CheckingAccount::withdraw> which
149 uses C<SUPER::withdraw> would not pass that extra argument
150 correctly. Whereas the method modifier version of would pass
151 all arguments along correctly automatically.
153 Just as with the first recipe, object instantiation is a fairly
154 normal process, here is an example:
156 my $savings_account = BankAccount->new(balance => 250);
157 my $checking_account = CheckingAccount->new(
159 overdraft_account => $savings_account
162 And as with the first recipe, a more in-depth example of using
163 these classes can be found in the F<t/002_basic.t> test file.
167 The aim of this recipe was to take the knowledge learned in the
168 first recipe and expand upon it within a more realistic use case.
169 I hope that this recipe has accomplished this goal. The next
170 recipe will expand even more upon the capabilties of attributes
171 in Moose to create a behaviorally sophisticated class almost
172 entirely defined by attributes.
180 Moose does not attempt to encode a class's is-a relationships
181 within the type constraint hierarchy. Instead Moose just considers
182 the class type constraint to be a subtype of C<Object>, and
183 specializes the constraint check to allow for subclasses. This
184 means that an instance of B<CheckingAccount> will pass a
185 C<BankAccount> type constraint successfully. For more details,
186 please refer to the L<Moose::Util::TypeConstraints> documentation.
194 =item Acknowledgement
196 The BankAccount example in this recipe is directly taken from the
197 examples in this chapter of "Practical Common Lisp". A link to that
200 L<http://www.gigamonkeys.com/book/object-reorientation-generic-functions.html>
206 Stevan Little E<lt>stevan@iinteractive.comE<gt>
208 =head1 COPYRIGHT AND LICENSE
210 Copyright 2006 by Infinity Interactive, Inc.
212 L<http://www.iinteractive.com>
214 This library is free software; you can redistribute it and/or modify
215 it under the same terms as Perl itself.